Conventionally, it has been proposed to form an optical waveguide on a silicon-on-insulator (SOI) substrate, in order to realize optical communication by use of the optical waveguide between function sections formed in this SOI substrate.
For example, Japanese Patent Application Publication Nos. 2002-14242 and 2002-323633 have disclosed that by using as a lower part clad an embedded silicon oxide film serving as an insulator layer of the SOI substrate, processing a silicon layer serving as a semiconductor layer of this SOI substrate to form a core, and depositing a silicon oxide film on the surface of this SOI substrate to form an upper part clad, an optical waveguide having a large specific refractivity difference is obtained.
In a case of forming an optical waveguide on the SOI substrate as described in the above-mentioned Japanese Patent Application Publication Nos. 2002-14242 and 2002-323633, the silicon layer at a side of the surface is utilized as the core of the optical waveguide. Therefore, it is impossible to form an electrical device at a portion in the SOI substrate where the optical waveguide is formed, which is made inconvenient when miniaturizing a system-on-chip (SOC) device that uses, for example, the SOI substrate.
Accordingly, it is conceived to employ an SOI substrate (hereinafter referred to as “double-structure SOI substrate”) in which insulator layers are formed doubly in the vicinity of its surface so that those two insulator layers and a semiconductor layer sandwiched by them may constitute an optical waveguide. FIG. 1 shows a configuration of the double-structure SOI substrate 50 in which an optical waveguide is formed.
This double-structure SOI substrate 50 has such a configuration that on a silicon substrate 51, a silicon layer (mono-crystal silicon film) 53 is formed via an insulation film (silicon oxide film) 52 and on this silicon layer 53, a silicon layer (mono-crystal silicon film) 55 is formed via an insulation film (silicon oxide film) 54.
In this case, the lower-layer insulation film 52 is formed so as to have a uniform distribution of depth, while on the other hand, the upper-layer insulation film 54 is formed so as to have a non-uniform distribution of depth so that the silicon layer 53 sandwiched between the insulation films 52 and 54 may have a thick portion formed along a predetermined path. Herein, since the refractive index of silicon (Si) is 3.5 and the refractive index of silicon dioxide (SiO2) is 1.5, the thick portion of the silicon layer 53 provides a core and the insulation films 52 and 54 corresponding to this thick portion provide clads, thereby forming an optical waveguide 56 along the predetermined path.
FIG. 2 shows a semiconductor device in which on the silicon layer 55 at the side of the surface of this double-structure SOI substrate 50, electrical devices 57, for example, MOS devices or the like, constituting a central processing unit (CPU), a memory, or the like are formed.
Since the insulation films 52 and 54 and the silicon layer 53 sandwiched between them constitute the optical waveguide 56 in the double-structure SOI substrate 50, the electrical devices 57 can be formed even on the silicon layer 55 right above this optical waveguide 56. Accordingly, by using this double-structure SOI substrate 50, the SOC device can be miniaturized.
In this double-structure SOI substrate 50, however, the upper-layer insulation film 54 has not uniform distribution of depth, so that the silicon layer 55 at a side of the surface is not uniform in thickness. Therefore, when forming, for example, MOS devices on the silicon layer 55, it is difficult to meet the characteristics of the MOS devices formed on the thin portion of the silicon layer 55 corresponding to the optical waveguide 56 with the characteristics of the MOS devices formed on the thick portion of the silicon layer 55 not corresponding to the optical waveguide 56. Further, mixture of the MOS devices having the different characteristics leads to complexity in a design of the electrical devices as a whole.